3083 - Assessment of Target Coverage and Tissue Sparing for Head and Neck Cancers Using Online Adaptive Stereotactic Body Radiation Therapy

Purpose/Objective(s):

Initial Head and Neck (HN) studies demonstrated the feasibility of utilizing stereotactic body radiation therapy (SBRT) which, while providing promising local control, can be associated with high-grade fatal toxicities. Online adaptive radiotherapy (ART), allowing fractional re-planning on new PTV and organ at risk (OAR) contours, provides an opportunity for increased coverage and enhanced tissue sparing. Ultra-fractionated SBRT has been associated with decreased toxicity risks and enables alignment with systemic therapy administration frequency. This study assesses ultrafractionation in a HN cohort using Ethos-driven ART for increased conformality and OAR sparing.

Materials/Methods:

Ten patients with malignancies involving the HN were treated with online adaptive SBRT. Each patient received 5 fractions using one of three regimes (36.25/45, 30/40, or 30/36.25 Gy) with fractions delivered every 8-20 days (average 12-days). Online ART fractions were delivered with a Hypersight-equipped Varian Ethos platform. OARs and targets were recontoured on the daily CBCT image before deciding whether to proceed with treatment on the scheduled or adapted plan. We assess PTV coverage across high and low dose volumes with D95% (%) and V95% (%) metrics between scheduled and adapted plans. Due to cohort heterogeneity, OARs for analysis were determined per patient as those meeting two criteria: those within 2cm of the low dose PTV, and with scheduled or adapted doses >80% of constraints.

Results:

Adapted fractions were chosen in 95.3% of cases, showing a mean increase in high dose PTV coverage from first to last fraction of 4.0% to 29.8% for D95% (%) and 3.5% to 19.5% for V95% (%). Similarly for the low dose PTV, mean coverage increased from 6.0% to 30.2% for D95% (%) and 3.5% to 17.5% for V95% (%) from the first to final fraction. If the adapted plan was chosen, OAR constraints were achieved in 84% of fractions compared to 33% for scheduled plans. Further, OAR sparing was improved in 49% of adapted fractions with an average dose reduction of 9% (1 Gy), with the remaining fractions seeing a 4% (0.4 Gy) increase in OAR dose to trade-off target coverage. Across all fractions, the average contouring, planning, and treatment times were 16.4, 6.6, and 10.2 minutes respectively, with an average total adaptive time of 33.2 minutes from contour generation to end of treatment.

Conclusion:

The data presented shows improved dosimetric coverage and increased tissue sparing using an ultra-fractionated adaptive online SBRT treatment regimen. This initial experience achieved favorable coverage improvement and enhanced sparing of proximal OARs thereby demonstrating the suitability of ART for extended temporal fractionation of HN SBRT. Further work will focus on the impact of clinical outcomes for such treatments.